The Book of Struggles is an extension to the All Us Gamers Inner Core Rules. It is optional, but recommended, to also use the On Aptitude extension.
Your character is in a conflict, a combat has broken out, you need to sneak past vigilant guards, you are attempting to change the King's mind, or your starship has to make it to the asteroid first in a race for control.
You can use the inner core to simply handle these as a set of discrete actions by your character, your fellow player characters and their opponents. However it is a richer experience to pit the actions of your side and your opponents side together. Then you have advances, and set backs, advantage gained and lost and complications arising from the struggle.
The mechanism is easy, based on struggle moments and the struggle matrix.
Once you have used this you won't want to go back to the old way of doing things.
(DriveThruRPG.com description https://www.drivethrurpg.com/product/319759/All-Us-Gamers--The-Book-of-Struggles)